1. Field of the Invention
The present invention relates to an overcurrent protection circuit and an overcurrent protection method, each of which prevents an overcurrent from flowing through a switching element.
2. Description of the Related Art
Generally, a switching control device, such as a switching power supply device, includes an overcurrent protection circuit for the purpose of protecting a switching element from destruction and deterioration. A general overcurrent protection method is a method for: detecting a current flowing through the switching element; when the detected current is higher than a predetermined threshold, determining that the current is an overcurrent state; and turning OFF the switching element. In such overcurrent protection circuit, false detection of the overcurrent may occur by a current flowing through a capacitance component, a noise, and the like at the time of switching of the switching element. To prevent such false detection, a predetermined period of time immediately after the switching is set as a period in which the detected current value is blanked out, that is, a so-called leading edge blanking period (hereinafter referred to as a “blanking period”). In a case where such blanking period is set, regardless of the detected current value, the switching element is definitely in an ON state during a minimum ON period including a delay time that is a time from when the blanking period terminates until when the switching element is actually turned OFF. Therefore, at the time of start-up or overload in which an output voltage of the switching power supply is low, in a case where the current flowing through the switching element increases during the minimum ON period, the current cannot be attenuated by turning OFF the switching element. On this account, the current may exceed a threshold of the overcurrent protection and keep on increasing for every minimum ON period, and this may cause the destruction and deterioration of the switching element. To solve such problems, for example, the overcurrent protection circuits described in Japanese Laid-Open Patent Application Publication No. 2007-20393 and U.S. Pat. No. 7,522,398 are known.
The overcurrent protection circuit disclosed in Japanese Laid-Open Patent Application Publication No. 2007-20393 will be briefly explained in reference to FIGS. 5 and 6. FIG. 5 is a schematic circuit diagram showing a flyback switching power supply device including a conventional overcurrent protection circuit. A power supply voltage VIN1 is connected to one end of a primary coil 901a of a switching transformer 901, and a current detector 930 and a switching element 920 are connected to the other end of the primary coil 901a in series. A capacitor 903 and a load 904 are connected to a secondary coil 901b of the switching transformer 901 via a diode 902 in parallel. An output voltage detector 905 detects an output voltage VOUT1. A current controller 950 performs ON-OFF control of the switching element 920 based on a detection result of the output voltage detector 905 such that the output voltage VOUT1 becomes a predetermined value. Moreover, in a case where the current detected by the current detector 930 exceeds the threshold, that is, the overcurrent state, the current controller 950 turns OFF the switching element 920, that is, performs overcurrent protection.
FIG. 6 is a schematic circuit diagram showing a configuration example of the current controller in the switching power supply device shown in FIG. 5. A gate drive signal DRIVE output from an RS flip-flop 959 to the switching element 920 is also input to a blanking pulse generator 960. The blanking pulse generator 960 generates a blanking signal BLANK after the switching element 920 is turned ON. The blanking signal BLANK contains a blanking pulse for setting a predetermined blanking period TBL1. A comparator 951 compares an output signal of the current detector 930 with a predetermined threshold ITH to determine whether or not the current flowing through the switching element 920 is the overcurrent state. An output of the comparator 951 is input to an AND gate 952 to which the blanking signal BLANK is input as the other input. Therefore, only in a period other than the blanking period, the comparator 951 can output a signal indicating that the current flowing through the switching element 920 is the overcurrent state. To be specific, since the output signal of the comparator 951 is blanked out during the blanking period, it is possible to prevent the false detection of the overcurrent caused due to a spike noise generated at the time of switching. When the signal level of an output signal OCU of the AND gate 952 becomes the H level, the RS flip-flop 959 is reset via an OR gate 958. With this, the switching element 920 is turned OFF, and this suppresses the overcurrent to the switching element 920.
Next, operations in the minimum ON period of the switching element 920 in the switching power supply device of the conventional example will be explained. Here, the minimum ON period of the switching element 920 is a period TMN1 obtained by adding a delay time TDL1 to the blanking period TBL1, the delay time TDL1 being a time from when the blanking period terminates until when the switching element 920 is actually turned OFF. When a minimum pulse detector 953 detects that the ON period of the switching element 920 has become the minimum ON period TMN1 for a predetermined number of times in a row, it sets a RS flip-flop 955 and switches the signal level of an output FAJD from the L level to the H level, the output FAJD being output to an oscillator 956. When the output FAJD to the oscillator 956 becomes the H level, the oscillator 956 lowers an oscillating frequency to delay the rising of the gate drive signal DRIVE (delay the ON operation of the switching element 920) by a predetermined time TDD1. When the signal level of the delayed gate drive signal DRIVE becomes the H level after the predetermined time TDD1, the RS flip-flop 955 is reset. Therefore, the oscillator 956 operates again at the oscillating frequency that is a frequency before the delay.
To be specific, in a case where the overcurrent protection circuit of the switching power supply device of the conventional example detects that the ON period of the switching element 920 has become the minimum ON period TMN1 for the predetermined number of times, it delays a switching cycle by the specific predetermined time TDD1. With this, a switching OFF period in which a switching current is attenuated is lengthened by the delay time TDD1. Therefore, even if the switching current continuously increases in the minimum ON period, it is possible to prevent the switching current from exceeding a current threshold and keeping on increasing.
Moreover, the overcurrent protection circuit disclosed in U.S. Pat. No. 7,522,398 is configured such that: in a normal operation, the switching element is repeatedly turned ON and OFF according to a predetermined cycle; the number of times the switching current flowing through the switching element has exceeded the current threshold is counted up; and the cycle of the turn-ON of the switching element is lengthened in accordance with the above number of times (the number of times the ON operation of the switching element is skipped at an ON timing of the switching element in the normal operation is increased). In such overcurrent protection circuit, when the switching current becomes the current threshold or lower, the switching element returns to the normal operation.